This invention relates to keyboard switch assemblies for inputting data to computers and other electronic devices. In particular, this invention relates to a mechanism for keyboard key switches that prevents key caps from tilting or rotating when the key caps are pressed by an operator.
A known mechanism for preventing key caps from tilting and rotating employs an element called a swing rod. Large key caps are assembled to respective key seats in several steps because a swing rod mechanism must be installed to the large key caps and their respective key seats. Small keys do not require a swing rod mechanism to maintain their orientations because forces generated by normal use are small enough to be resisted by guides in an incorporated electrical switch. The absence of a swing rod mechanism permits assembly of small key caps to respective key seats in a single-step operation. This results in a substantial saving in manufacturing cost.
A prior art key switch assembly employing swing rods is shown in FIGS. 13a to 13c, in which a key switch assembly 40, according to the prior art, employs a coil spring (not shown) to urge a key cap 27 upward from a key seat 24.
Referring to FIG. 13d, a swing rod 2 has two arms 2c and a center portion 2a that define three sides of a rectangle. Each of arms 2c is connected to a tip 2b that extends partly along a fourth side of the rectangle. Tips 2b extend inward from opposite ends of the fourth side of the rectangle.
Referring again to FIGS. 13a-13c, tips 2b are rotatably anchored by a pair of bearings 5 to key seat 24. Center portion 2a is rotatably held at two ends in a pair of grips 3 on key cap 27. Center portion 2a extends nearly an entire length of key cap 27.
Without swing rod 2, a lateral or uneven force imposed on key cap 27 tends to tilt it. Key cap 27 would then bind in a switching mechanism (not shown) when tilted. Such binding makes it difficult to stroke key cap 27 smoothly. With swing rod 2, a force tending to tilt key cap 27 forces downward a first end of key cap 27. A resulting downward movement of the first end of key cap 27 forces down a first end of center portion 2a and a first one of arms 2c, contiguous with the first end of center portion 2a. At the same time, tip 2b of the first one of arms 2c is prevented from shifting downward by its anchoring block 19. Thus, the first arm 2c rotates center portion 2a about an axis of center portion 2a. Rotation of swing rod 2 rotates a second arm 2c. Since the second arm 2c is held down by another anchoring block 19, the second arm 2c forces downward a second end of center portion 2a , contiguous with the second arm 2c. The second end of center portion 2a moves key cap 27 down with it because center portion 2a is coupled at the second one of its ends to key cap 27 by one of grips 3.
Swing rod 2 rotates every time key cap 27 is pressed, transmitting any downward forces on one edge of key cap 27 to an opposite edge of key cap 27. Thus, initial tilting of key cap 27 about an axis perpendicular to both the axis of center portion 2a and a direction of stroke, generates an opposing moment caused by the rotation of swing rod 2. The opposing moment tends to prevent key cap 27 from tilting further.
Referring to FIG. 13a, during assembly, key cap 27 is mounted onto key seat 24 as follows. First, tips 2b of swing rod 2 are placed on top of bearings 5, each of which has an access groove facing upwardly. Tips 2b are then pressed into bearings 5. Next, center portion 2a is fitted into grips 3 of key cap 27.
Referring to FIG. 13b, swing rod 2 and key cap 27 are next pivoted about bearings 5 toward key seat 24 bringing them to their home operating positions.
Referring to FIG. 13c, swing rod 2 and key cap 27 are shown in their home operating positions.
During assembly of the prior art key switch of FIGS. 13a-13c, center portion 2a and tips 2b of swing rod 2 are press-fitted to key cap 27 and key seat 24, in separate steps. It is difficult to keep center portion 2a from detaching from grips 3 when key cap 27 is moved to its home operating position as shown in the sequence of FIGS. 13a to 13c. According to the prior art arrangement, it is almost impossible to mount all key caps 27 of a keyboard to corresponding key seats 24 in a single-step operation. Therefore, efficiency and cost-effectiveness of the assembly process are sacrificed.
It would be desirable to have a keyboard assembly which permits single-step assembly of large key switches that require swing rod mechanisms. Single-step assembly of such large key switches permits the assembly of all keys of an entire keyboard, including small and large keys, in a single step.